Dinorwic Quarry, the second largest producer, had a lot of similarities to Penrhyn. It was situated only 5 miles away and used the same gallery system for slate extraction. The slate vein at Dinorwic is nearly vertical and lies at or near the surface of the mountain, allowing it to be worked in a series of stepped galleries. The quarry covered 324 hectares and was divided into galleries that were 20 to 40 metres deep. The width of the gallery should be three quarters of the depth, although this requirement was rarely applied.
A tramway was first completed to Port Dinorwic (now Y Felinheli ) in 1824 and in 1842 it was replaced by the four foot gauge Padarn Railway. At the quarry itself there were three main incline runs, A, B and C, and the highest working level was over 1500 feet above sea level. In many ways Dinorwic was always second to Penrhyn and never achieved its efficiency or productivity. There was much careless dumping of waste rock and never enough modernisation.
Dinorwic finally closed in 1969. The quarry workshops were preserved and they are now the home of the Welsh Slate Museum. Amongst many other attractions are three rebuilt quarrymen's cottages from Blaenau Ffestiniog and a working table incline. To get an impression of its huge size one can view Dinorwic Quarry from Llanberis.
Conditions at the quarry were extremely harsh. Facilities like canteens, places to wash and keep clothes dry were minimal, as were the quarrymen’s wages. Workers had to take a five-year apprenticeship to become a fully-fledged quarryman. It may have been skilled work but it was also dangerous, dirty and unhealthy. Accidents were usually caused by a fall of rock or fragile ropes breaking. The Quarrymen were responsible for all their own ropes, hammers and chisels, which they had to pay for to be sharpened out of their wages. The industrial disease, silicosis, was rife at Dinorwig because of the high dust content of the slate mined there.
Conditions at Dinorwig were rapidly getting worse and in the 1885. Fifty-three men were suspended from working because ten men had broken a local rule. A mass meeting was held at Craig yr Undeb where votes of no confidence were passed in the manager, John Davies, as well as the chief manager, Walter Warwick Vivian. Vivian had no experience of the quarrying industry. Both had their training in the hard world of business, and were not in the least sympathetic to inefficient customs and practices. During the 1885-86 strike, there is no doubt that it was Vivian who was in charge at Dinorwig. A deputation was elected to visit G.W.D. Assheton Smith, but his response was to inform his workforce to remove their work tools and barracks furniture out by the last day of October. The lock out lasted until Saint David's Day, 1886.
The slate industry reached its peak towards the end of the nineteenth century. Then in 1900 came a long and bitter strike which has gone down in history as the most difficult dispute in welsh industrial life. The quarrymen at Dinorwig had wanted the North Wales Quarrymen’s Union to represent them in negotiations over pay and conditions. But after three long years they failed to achieve this end. Even today, the descendants of the quarrymen involved in the struggle recall the bitterness felt by the workers at the time.That strike, in many ways represented a turning point for the state industry. Alternative roofing materials were sought and the demand for slate never again recovered its former level.