With slowly advancing enamel caries and rapid involvement of dentin, the type of dentin formed is:

Sclerotic dentin  Dentin that forms in response to stimuli such as aging or mild irritation (slow advancing caries). When responding to initial caries demineralization events, crystalline material precipitates in intratubular and intertubular dentin. Sclerotic dentin walls off a lesion by blocking (sealing) the dentinal tubules. This zone can be seen even before the demineralization reaches the dentin and it may not be present in rapidly advancing lesions. Clinically, sclerotic dentin is dark and harder than normal dentin. Sturdevant's operative dentistry 7th edition page 43