144 Professor Keilhau on Contact Products. 



in its development, but may be rendered entirely false. This 

 has taken place to the fullest extent with respect to the ques- 

 tions which have arisen as to contact productions. It was 

 firmly believed that here the views of the volcanists could 

 alone satisfy the requirements of chemistry ; and as it was 

 a fundamental rule that these should and must be satisfied, 

 so it followed that these products should and must owe their 

 origin to processes put in operation by the great agent of the 

 volcanists. Some phenomena actually produced by heat ex- 

 isted, which, on a superficial consideration, might answer as 

 analogies, and in this way we now find elevated to the rank of a 

 doctrine in our science what should never have been more than 

 a mere provisional opinion. This mode of at once regarding 

 as certain what should have been based on a complete investi- 

 gation of all the facts, has also here produced its usual effects. 

 No attention has been bestowed on a multitude of facts and 

 special circumstances which are most intimately related to the 

 subject, while in the descriptions other facts have been adapted 

 to that doctrine, nay, under its influence even ornamented 

 with additions, which it would give subsequent observers some 

 trouble to rediscover. That, for example, the changes of the 

 kind we are now discussing, which have been instanced by 

 keen partisans as consisting of actual conversions into melted 

 masses and slags, will in so far be shewn by later observers to 

 be entirely incorrect, is a conviction which I cherish, and 

 which, trusting to the impartial decision of futurity, I am not 

 afraid to express. 



Let us suppose that the object of study be those changes 

 that have occurred in certain rocks near a bounding mass, 

 and which consist especially in this, that the rock has become 

 harder, and has acquired a considerable quantity of silica ; and 

 that, at the same time, it matters not whether sooner or later, 

 the knowledge of the following facts has been attained : 



1. In the Isle of Portland, there rests immediately on a 

 stratum belonging to the oolite group, a bed which contains 

 a quantity of trunks of trees completely converted into a sili- 

 ceous mass. Round these the rock is also harder and contains 

 more silica, than where it is not in the vicinity of the trees. 



2. In the Peninsula of Melazzo, a very new (" quaternary") 



