12 Analyses of Igneous Rocks [210 



I might go on and enumerate in much greater detail the 

 innumerable questions which such an area as Maryland pre- 

 sents to the geologist. We have been engaged, as I stated 

 earlier, for over twenty years in trying to reach a solution of 

 some of these problems, but our successors will, I am sure, 

 find enough to keep them fully occupied for another genera- 

 tion if it is not vouchsafed for us to keep actively employed 

 in studying them during that time. The question may be 

 asked, is often asked by the custodians of the public funds, 

 will this work never end ? But I must answer no, not as long 

 as there is a science of geology worthy of the name. 



THE USE OF AVERAGE ANALYSES IN DEFINING 

 IGNEOUS ROCKS 



By EDWARD B. MATHEWS 



There is usually associated with the consideration of rock 

 names and their meanings some attempt to represent the 

 characteristic chemical composition connoted by the name. 

 The methods employed usually /consist of the presentation 

 either of a series of analyses of individual rocks with little or 

 no discussion of their meaning or of an arithmetical mean of 

 a varying number of such analyses in the form of an " aver- 

 age " analysis also without discussion of the departures from 

 such averages which may be shown by the analyses on which 

 this " average " is based. Neither of these methods is very 

 satisfactory for teaching or textbooks. 



The presentation in columnar form of a series of analyses 

 each of which includes from eight to fifteen determinations 

 bewilders the student who seldom stops to consider just what 

 the variations amount to either absolutely or relatively, or 

 what relations the variants bear to the general type. This 

 method of enumerating the actual composition of individual 

 rocks may be eminently proper in a Handbook but fails of its 

 purpose in a textbook. 



