V 
1880.) . 855 (Diller. 
THE FELSITES AND THEIR ASSOCIATED ROCKS NORTH OF 
Boston. By J. S. DILurrR. 
It is to be greatly regretted that there is no accurate map of | 
Boston and vicinity on a sufficiently large scale to be used to advan- 
tage in constructing a geological map. Mr. W. O. Crosby in pre- 
paring his contributions to the Geology of Eastern Massachusetts 
uses a map based on Mr. H. F. Walling’s surveys, having a 
scale of an inch toa mile. It is not sufficiently large to allow the 
ordinary basaltic dikes and the smaller areas of some of the other 
rocks to be represented. 
In my work I have used D. G. Beer’s maps of the towns about 
_ Boston and reduced them all to the scale of a thousand feet to an 
inch. These maps, like others of this region, when compared with the 
accurate work of some of the civil engineers of the towns mapped, are 
found to be very erroneous, but they are, on the whole, the best 
maps that could be obtained. 
The felsites occur in Medford, Malden, Melrose, Wakefield, 
Saugus, Lynn, and Marblehead Neck. Only a portion of these 
towns has already been mapped, and several years will be required to 
complete the work. 
I am greatly indebted to Mr. W. O. Crosby for an advance sheet 
of his “ Geological Map of Boston and Vicinity.” This map has been 
of very great service to me in the field by showing the general distri- 
bution of the formations. On account of the many errors which 
it contains the ‘Centennial Map ” should be supplanted as quickly as 
possible by another map giving accurately the detailed distribution of 
the various rocks. 
The rocks associated with the felsites are diorite, a stratified group 
consisting of quartzite and slates, granite, breccia, conglomerate 
slate, diabase and melaphyre. These rocks have not been studied to 
_ any considerable distance from their junction with the felsite. It has 
been thought best to describe them in their order of geological suc- 
1 Last year I took a course of instruction in Microscopic Lithology under Mr. 
M. E. Wadsworth at the Museum of Comparative Zoology in Cambridge. He 
assigned to me the Malden Highlands as the region, the lithology and geology of 
which I was to unravel. It was soon found advisable to enlarge the field so as to 
include all the felsites north of Boston. This paper is merely a preliminary 
report upon some of the work already accomplished, and the general statements 
which it contains refer only to the region under consideration. In a few years. I 
hope to present a complete report upon the field work as well as upon the 
microscopic and some of the chemical characters of the rocks studied. 
