1884.] 17 [Crosby. 



the drillings seems to indicate a variety of materials, and is 

 favorable to the view that the rock is conglomerate. And the 

 material removed by the sand pump each morning before drilling 

 commenced, and which consists of fragments that had become 

 detached from the side of the well during the night, includes 

 many small, well-rounded pebbles of quartzite, trap, etc., just 

 those materials which are most abundant in the Roxbury pud- 

 ding stone. No pebbles were obtained from a less depth than 

 2300 feet ; and the indications are plain that at that depth the 

 well reached the beds of passage between the slate and underly- 

 ing conglomerate. 



The well of the Boston Gas Company, on Causeway Street, is 

 1750 feet deep ; and was bored almost wholly in slate, the last 

 fifty feet only being in a harder rock which is quite possibly con- 

 glomerate, although, having seen no samples of the borings, I can- 

 not speak positively on this point. Dr. T. Sterry Hunt has called 

 attention to the fact that in the composition of the water of this 

 well we have a plain indication that the slates yielding the water 

 are of Cambrian age, and, therefore, probably synchronous with 

 the Brain tree slate. 



Some of the larger dikes of diabase cutting through the slates 

 include fragments of quartzite similar to that entering so largely 

 into the composition of the conglomerate. Some of these frag- 

 ments may have come, as Dr. Wadswoi th suggests, from a for- 

 mation of quartzite underlying the slate ; but others, it is clear, 

 cannot have been derived directly from this source, because they 

 are well rounded or water-worn pebbles such as could only have 

 come from a formation of conglomerate below the slate. 



The angular fragments of quartzite enclosed in dikes travers- 

 ing the slate and the fact that this is by far the most abundant 

 material in the conglomerate, which is often entirely composed 

 of pebbles of quartzite, while the only quartzite of similar char- 

 acter now exposed on the surface is limited to a small area in 

 Wellesley and Natick, lend much probability to the view that 

 the conglomerate and slate rest in part upon quartzite. Accord- 

 ing to Dr. Wadsworth, this conclusion is confirmed by borings 

 in various parts of the Boston basin ; and it is only in this way 

 that we can find an adequate source for the pebbles in the con- 

 glomerate. But, if the slate underlies the conglomerate, then all 



PROCEEDINGS B. 8. Jf. H. VOL. XXIII. 2 OCTOBER, 1884. 



