THE 
AMERICAN GEOLOGIST. 
Vol. XXVIII. SEPTEMBER, 1901. No. 3. 
THE BASIC ROCKS OF NORTHEASTERN MARY- 
LAND, AND THEIR RELATION TO 
THE GRANITE. 
Arthur Gray Leonard, Des Moines, Iowa. 
PLATES X\^-XIX. 
INTRODUCTION.* 
This paper contains the results of the study of a Hmited area 
of igneous rocks in which are found a number of different tvpes 
ranging- all the way from acid to ultra-basic. It is intended to 
show that these types are intimately associated in their geologi- 
cal occurrence and closely related in composition ; that many of 
the types graduate into others by intermediate varieties, and 
that they probably represent facies of one original magma. 
The study of these eruptive rocks was suggested by the 
late professor G. H. Williams, and the field work was com- 
menced and carried on during the greater part of the month of 
June, 1893. At the same time the investigation of the granites 
lying just to the south was taken up by G. P. Grimsley and the 
results of his studies have already been published. f Accom- 
panying his paper is a map, on which are represented not 
only the granites, but also the basic rocks lying to the 
north. The data concerning the distribution of the latter 
were furnished by the present writer and represent the views 
*The writer is under special obligation to Dr. IL F. Reid, of the Johns 
Hoplfins I'niversity. for the jjhotomicrographs in tnis paper, wliicli were 
made in the geoloKical laboratory of the I'niversity. He is also much indebted 
to professor W. l^. Clark and Dr. K. B. Mathews, of the same I'niversity, for 
helpful suggestions and for assistance rendered in many ways. 
tThe granites of Cecil county in northeastern Maryland. Jour. Cincinnati 
Soc. Nat. Hist., A'ol. 17, pp. 59-G7 ; 78-114. Cincinnati, 1894. 
