Components of an Igneous Rock— Williams 35 
of the minerals, the more nearly ideal is the sample for the 
purpose in view. More important still, perhaps, is the re- 
quirement that the minerals themselves be such that each 
and every grain can be readily and certainly recognized at a 
glance under the microscope. The misidentification of a 
grain, or of a few grains in a field, especially if the rock be 
coarse textured, is certain to afiford results that are more 
or less incorrect. 
After some preliminary and unsatisfactory experiments 
with the Palisade diabase from New Jersey and a nephelite 
syenyte from Red Hill, New Hampshire, the rock finally 
selected was the pink granite from Westerly, Rhode Island.* 
The Westerly granite is moderately coarse-grained as a rule, 
and unusually so in places. The individual grains of the 
principal minerals range from 1.5 to 5 mm. in diameter. 
Professor J. F. Kemp in the paper cited gives a com- 
plete description of the rock and enumerates the essential 
and accessory minerals which compose it. Under the micro- 
scope the following minerals are recognizable in practically 
every field. Orthoclase, often showing the reticulated struc- 
ture of microcline, a plagioclase, near oHgoclase, quartz, 
magnetite. A little muscovite and apatite occur, but so far 
as their presence is concerned in the percentage composi- 
tion of the rock as a whole, can be neglected. Biotite is a 
normal constituent but in the sample worked with was pres- 
ent in only very subordinate amount. Other more rare ac- 
cessories occur but are neglected in the present study. 
The anhedra of quartz and the feldspars are predomi- 
nantly distinct, one not including the other as a rule. In 
some slides, however, the quartz is sometimes broken up into 
smaller rounded grains which are frequently included in the 
feldspars. The boundaries in these cases are not so sharply 
defined and the necessity of taking into account numerous 
small particles of one mineral in a groundmass, as it were, 
of a large grain of another, involves of course chance of in- 
accuracy. As with the Palisade diabase and the New Hamp- 
shire syenyte, there arose the question of being able to 
always distinguish between the feldspars. It was believed 
*J, F. Kemp, Granites of Southern Rhode Island, with Observa- 
tions on Atlantic Coast Granites in General. Bu// Geol. Soc. Amer., Vol 
10. 1899, p. 368. 
