516 ME. E. E. L. DIXON AND DR. A. VAUGHAN ON [NOV. I9II, 



occurrence in the Avonian than the preceding rock-type, being 

 recorded only from Km in a few localities. Their unusual colour 

 points to deposition in comparative isolation. Furthermore, the 

 fact that in other formations they are often associated with con- 

 tinental deposits suggests proximity to land ; but it remains to be 

 seen whether they have been formed under other conditions of 

 isolation as well. 



Calcite -mudstones and similar rock-types. — The 

 second of the groups of rock-types distinctive of Modiola phases is 

 much more widespread than either of the preceding types ; its 

 members occur in all such phases as are known and at many 

 places in each. They are compact 1 limestones, dolomites, and 

 argillaceous rocks which, under the microscope, are seen to be com- 

 posed, essentially, of exceedingly fine-grained, calcareous (that is, 

 either calcitic or dolomitic), argillaceous, and siliceous material, and 

 evidently represent, in most cases at least, consolidated muds 2 of 

 various compositions. For this reason, the calcareous members will 

 be spoken of as calcite-mudstones or dolomite-mudstones, 3 in order 

 to distinguish them from limestones or dolomites of standard types 

 The calcite-mudstones include the rocks known as 'chinastone-lime- 

 stones,' so called because their compactness and white weathered 

 surface impart to them a porcellanous appearance. Most of the 

 rocks are evenly bedded, bat some have concretionary forms. The 

 calcite- and dolomite-mudstones are either homogeneous and more 

 or less conchoidal in fracture, or are laminated and present a slabby 

 or platy" fracture ; the laminae, which, in places, are extremely thin, 

 are either persistent or lenticular — wedge-bedded on a minute 

 scale. The homogeneous and the laminated types are equally fine 

 in grain. Similarly, the argillaceous rocks may be either clay-mud- 

 stones, — rocks to which the term ' mudstone ' is ordinarily applied — , 

 shales, or paper-shales. Under the microscope the bedding in the 

 case of the laminated mudstones, calcareous or otherwise, is clearly 

 shown, even in the absence of fossils, by the layers, which differ 

 greatly one from the other in composition and appearance ; but in 

 sections of the non-laminated mudstones it cannot, as a rule, be 

 made out without the help of fossils. Where largely calcareous, 

 such mudstones might be taken for travertine ; but that they have 

 been deposited in the form of mud, whether detrital or chemically 

 precipitated, is shown by the fact that, where the material has partly 



1 That is, amorphous-looking, and, in fact, often called ' amorphous ' though 

 the constituent grains possess crystal-structure. 



2 This statement is based on an examination, still in progress, of various 

 limestones and dolomites that occur in Modiola phases. It remains to be seen 

 whether it is universally true of the compact layers ; in some cases, as, for 

 example, that of landscape-marbles, an origin as travertine by direct accretion 

 from solution has previously been suggested. 



3 It is possible that the dolomite-mudstones are calcite-mudstones dolomitized 

 without change of structure, but the balance of evidence at present available 

 suggests that they have been deposited as dolomite-mud. 



